The invention relates to liquid flowrate monitoring, and more particularly to an improved monitoring system which chooses one of a plurality of flowrate determination methods, depending upon which is more accurate for conditions prevailing at the time a determination is to be made.
Increasing activity in water pollution control and a growing scarcity of irrigation water have created a need for open-channel flow monitoring instruments with improved accuracy and reliability. Previously available instrumentation has employed either critical-flow or velocity-area techniques. Flowmeters using critical-flow techniques have delivered 95% to 98% accuracy when properly installed, but have been rendered useless by submergence. Velocity-area type flowmeters have been able to operate in both submerged and free-flowing conditions, but with much larger errors, by factors of up to 10 compared with critical-flow meters, due primarily to velocity profile related factors.
In a large number (probably the majority) of applications, the condition of submergence is occasional, with critical flow conditions occurring most of the time. A critical-flow instrument determines flowrate from a measurement of liquid depth, so the submergence is read and integrated as a very large flowrate, even if the liquid is not moving and the flowrate is actually zero. The velocity-area meter can measure the flow under submerged conditions, but displays very poor measuring accuracy because of the difficulty in measuring the small velocities inherent in the low flowrates, and also due to errors resulting from uncertainties and variation in the velocity profile.